Field effect transistor

ABSTRACT

An insulating film is provided over one surface of a first semiconductor layer including a first oxide semiconductor including indium as a main component, and a second semiconductor layer including an i-type second oxide semiconductor is provided in contact with the other surface. The energy difference between a vacuum level and a Fermi level in the second oxide semiconductor is larger than that in the first oxide semiconductor. In the first semiconductor layer, a region in the vicinity of the junction surface with the second oxide semiconductor which satisfies the above condition is a region having an extremely low carrier concentration (a quasi-i-type region). By using the region as a channel, the off-state current can be reduced. Further, a drain current of the FET flows through the first oxide semiconductor having a high mobility; accordingly, a large amount of current can be extracted.

TECHNICAL FIELD

The present invention relates to a field effect transistor (FET)including an oxide semiconductor.

BACKGROUND ART

A field effect transistor (FET) is a device in which regions called asource and a drain are provided in a semiconductor, each of the regionsis provided with an electrode, potentials are supplied to theelectrodes, and an electric field is applied to the semiconductor withthe use of an electrode called a gate through an insulating film or aSchottky barrier so that the state of the semiconductor is controlled,whereby current flowing between the source and the drain is controlled.As the semiconductor, Group IV elements (also referred to as Group 14elements) such as silicon and germanium, Group III-V compounds such asgallium arsenide, indium phosphide, and gallium nitride, Group II-VIcompounds such as zinc sulfide and cadmium telluride, and the like canbe given.

In recent years, FETs in which oxides such as indium oxide (PatentDocument 1), zinc oxide (Patent Documents 2 and 4), and an indiumgallium zinc oxide-based compound (Patent Document 3) are used assemiconductors have been reported. In a FET including such an oxidesemiconductor, relatively high mobility can be obtained, and such amaterial has a wide bandgap of greater than or equal to 3 eV; therefore,application of the FET including an oxide semiconductor to displays,power devices, and the like is examined.

To be specific, it is reported that the field effect mobility of a FETincluding zinc oxide or an indium-gallium-zinc-oxide-based compound is20 cm²/Vs at most, while the field effect mobility of a FET includingindium oxide as a main component is 50 cm²/Vs or higher. It isempirically clear that a higher field effect mobility can be obtainedwith a higher ratio of indium in an oxide.

In general, an oxide semiconductor including zinc or indium as a maincomponent (here, “the main component” refers to an element accountingfor 50 at. % or more of all elements having an atomic number of 11 ormore in the oxide semiconductor) and showing a p-type conductivity hasnot been reported so far. Accordingly, a FET using a PN junction like aFET including silicon has not been reported. As disclosed in PatentDocuments 1 to 4, a metal-semiconductor junction in which a conductiveelectrode is in contact with an n-type or i-type (in this specification,“an i-type semiconductor” refers to a semiconductor having a carrierconcentration of lower than or equal to 1×10¹⁴ /cm³) oxide semiconductorhas been used for forming a source and a drain.

FIG. 7A illustrates an example of a conventional FET including an oxidesemiconductor. Here, a gate insulating film 14 is provided in contactwith one surface of a semiconductor layer 11 including an oxidesemiconductor, and a gate 15 is provided over the gate insulating film14. A source electrode 13 a and a drain electrode 13 b are provided onthe other surface of the semiconductor layer 11.

The thickness of the semiconductor layer 11 has not been particularlyconsidered in many cases. In addition, as a material of the gateinsulating film 14, silicon oxide, silicon nitride, or the like has beenused, and the thickness of the gate insulating film 14 has not beenconsidered particularly as well. A material of the source electrode 13 aand the drain electrode 13 b has not also been considered particularly,and titanium, molybdenum, and the like have been reported.

In practice, a protective insulating film 16 is provided in contact withthe semiconductor layer 11 as illustrated in FIG. 7B. As a material ofthe protective insulating film 16, a material that can be used as amaterial of the gate insulating film 14 is used.

In a FET, it is generally preferable that an ohmic contact is formed ina contact portion between a source electrode and a semiconductor layeror a contact portion between a drain electrode and a semiconductorlayer. For this purpose, the material of the source electrode 13 a andthe drain electrode 13 b is preferably a material having a work functionlower than the electron affinity of an oxide semiconductor that is usedfor the semiconductor layer 11. For example, work functions of titaniumand molybdenum are lower than the electron affinity of indium oxide(approximately 4.8 eV) and thus preferable in terms of forming an ohmiccontact.

Further, in portions where the metal is in contact with thesemiconductor layer 11, electrons are injected from the metal to thesemiconductor layer 11, so that the concentration of electrons in thesemiconductor layer 11 is increased, which couples regions having highelectron concentration together particularly in the case of ashort-channel FET having a channel length (the distance between thesource electrode 13 a and the drain electrode 13 b) of 0.3 μm or less,and is a factor of a reduction of FET characteristics (e.g., a negativeshift of the threshold voltage, an increase in S value, and a phenomenonin which current flows between a source and a drain in an off state(off-state current)).

In a FET in which a source and a drain are formed using ametal-semiconductor junction, a higher carrier concentration of asemiconductor causes a larger off-state current. In other words, evenwhen the source-gate voltage (hereinafter referred to as a gate voltage)is 0 V, a substantial amount of current (hereinafter referred to as adrain current) flows between the source and the drain (this FETcharacteristic is called “normally on”). For this reason, it is expectedthat the off-state current is reduced by reducing the concentration ofcarriers in the semiconductor so that the semiconductor is formed to bean i-type semiconductor and that the drain current at a gate voltage of0 V is 1×10⁻⁹ A or lower, preferably 1×10⁻¹² A or lower, and furtherpreferably 1×10⁻¹⁵ A or lower.

However, oxygen deficiency is likely to be caused in indium oxide or anoxide semiconductor including indium as a main component, and it hasbeen difficult to set the carrier concentration to 1×10¹⁸/cm³ or lower.Accordingly, a FET including an oxide semiconductor including indium asa main component has high mobility but is normally on, and this tendencybecomes more significant as the concentration of indium becomes higher.For example, in the case of using indium oxide, the drain current cannotbe 1×10⁻⁹ A or lower unless the gate voltage is set to be −10 V orlower.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    H5-251705-   [Patent Document 2] United States Patent Application Publication No.    2005/0199879-   [Patent Document 3] United States Patent Application Publication No.    2007/0194379-   [Patent Document 4] United States Patent Application Publication No.    2009/0283763

DISCLOSURE OF INVENTION

The present invention achieves at least one of the objects describedbelow. An object is to provide a FET which includes an oxidesemiconductor including indium as a main component and has a highmobility and a normally off characteristic (the threshold voltage is 0 Vor higher) or a characteristic close to the normally off characteristic.Another object is to provide a FET including an oxide semiconductor inwhich the percentage of indium to all elements other than oxygen is 50%or more and preferably 75% or more and having a normally offcharacteristic or a characteristic close to the normally offcharacteristic. Another object is to provide any one of the followings:a novel FET having a metal-semiconductor junction and including indiumas a main component, a novel semiconductor device having ametal-semiconductor junction and including indium as a main component, amanufacturing method of the novel FET, and a manufacturing method of thenovel semiconductor device.

Note that the description of these objects does not disturb theexistence of other objects. Note that one embodiment of the presentinvention does not necessarily achieve all the objects listed above. Inaddition, other objects will be apparent from and can be derived fromthe description of the specification, the drawings, the claims, and thelike.

The present invention will be described below; terms used in thisspecification are briefly explained. Terms which are not defined in thisspecification (including terms used for science and technology, such astechnical terms or academic terms) can be used as the terms havingmeaning equal to general meaning that an ordinary person skilled in theart understands. It is preferable that terms defined by dictionaries orthe like be construed to have meanings consistent with the background ofrelated art. One embodiment of the present invention should not beconstrued as being limited by the technical terms.

As for a source and a drain of a FET in this specification, a terminalsupplied with a higher potential is referred to as a drain and the otherterminal is referred to as a source in an n-channel FET, and a terminalsupplied with a lower potential is referred to as a drain and the otherterminal is referred to as a source in a p-channel FET. In the casewhere the same potential is supplied to the two terminals, one of themis referred to as a source and the other is referred to as a drain. Inaddition, the terms “first electrode” and “second electrode” are usedinstead of the terms “source electrode” and “drain electrode” in somecases. In that case, the names are not changed depending on the level ofa potential.

Further in this specification, “a main component” refers to an elementaccounting for 50 at. % or more of all elements having an atomic numberof 11 or more in the target object. For example, in the case of acompound whose nominal composition formula is represented byGa₃Al₂In₅O₁₂N₂, oxygen (O) is the element accounting for the largestportion of the compound; however, oxygen has an atomic number of 8 andis not regarded as a main component. Similarly, nitrogen (N) does notserve as a main component. Gallium (Ga), aluminum (Al), or indium (In)can serve as a main component, and the ratio thereof is as follows:Ga:Al:In=3:2:5. In other words, the percentages of gallium, aluminum,and indium to the all elements of the possible main components are 30at. %, 20 at. %, and 50 at. %, respectively. Therefore, in the abovedefinition, indium is a main component and gallium and aluminum are notmain components.

The nominal composition ratio, a compound having the nominal compositionratio, and the nominal chemical formula (or the nominal compositionformula) refer to a ratio of elements that exist in a region, a compoundhaving such a ratio of elements, and a chemical formula based on such aratio of elements, respectively and are not the terms in considerationof the microscopic or local ratio, the chemical meaning, stability, andthe like. In the above example, those elements may form a solid solutionat the above ratio, or one molecule of Ga₂O₃, two molecules of AlN, twomolecules of In₂O₃, and one molecule of InGaO₃ may form a mixed crystalor a mixture.

Note that in this specification and the like, terms such as “first”,“second”, and “third” are used for distinguishing various elements,members, regions, layers, and areas from others. The terms such as“first”, “second”, and “third” therefore do not limit the number or theorder of the elements, members, regions, layers, areas, and the like.

An embodiment of the present invention is a FET which includes a firstsemiconductor layer including a first oxide semiconductor includingindium as a main component, a second semiconductor layer including ani-type second oxide semiconductor having a wider bandgap than the firstoxide semiconductor and is provided in contact with one surface of thefirst semiconductor layer, a conductive layer functioning as a gateelectrode and adjacent to the other surface of the first semiconductorlayer, and an insulating layer also functioning as a gate insulatingfilm provided between the conductive layer and the first semiconductorlayer. An energy difference between a vacuum level of the second oxidesemiconductor and a Fermi level of the second oxide semiconductor islarger than an energy difference between a vacuum level of the firstoxide semiconductor and a Fermi level of the first oxide semiconductor.

Another embodiment of the present invention is a FET which includes afirst semiconductor layer including a first oxide semiconductorincluding indium as a main component, a second semiconductor layerincluding an i-type second oxide semiconductor including gallium as amain component and having a wider bandgap than the first oxidesemiconductor and is provided in contact with one surface of the firstsemiconductor layer, a conductive layer also functioning as a gateelectrode and adjacent to the other surface of the first semiconductorlayer, and an insulating layer functioning as a gate insulating filmprovided between the conductive layer and the first semiconductor layer.

Another embodiment of the present invention is a FET which includes afirst semiconductor layer including a first oxide semiconductorincluding indium as a main component, a second semiconductor layerincluding an i-type second oxide semiconductor in which the percentageof gallium to all elements other than oxygen is 80% or more in contactwith one surface of the first semiconductor layer, and a conductivelayer functioning as a gate electrode and adjacent to the other surfaceof the first semiconductor layer. An insulating layer also functioningas a gate insulating film is provided between the conductive layer andthe first semiconductor layer.

In each of the above embodiments, the thickness of the firstsemiconductor layer is preferably more than or equal to 0.1 nm and lessthan or equal to 100 nm for a reason described later. The thickness ofthe second semiconductor layer is preferably more than or equal to 10 nmand less than or equal to 100 nm.

Further, an insulating film which includes an oxide including aluminumas a main component and having a bandgap of 8 eV or more may be providedin contact with a surface of the second semiconductor layer that isopposite to a surface in contact with the first semiconductor layer.

Further, an insulating film which includes an oxide including aluminumas a main component and having a bandgap of 8 eV or more may be providedin contact with a surface of the first semiconductor layer that isopposite to the surface in contact with the second semiconductor layer.

As the first oxide semiconductor, an oxide including indium as a maincomponent can be used. For example, a material which includes elementsselected from indium, gallium, aluminum, zinc, and oxygen at 90 at. % ormore, preferably 95 at. % or more of the all elements and whose nominalcomposition formula is represented by In_(a)Ga_(b)Al_(c)Zn_(d)O_(e)(here, a+b+c+d=2, a≧1, 2.5<e<3.5) may be used. Note that in order toincrease the mobility, the concentration of indium is preferably highand a is preferably larger than 1.6. For the same purpose, theconcentration of gallium is preferably higher than that of aluminum, andb is preferably larger than c, further preferably larger than 10c.

As the first oxide semiconductor, an oxide semiconductor includingindium as a main component and including oxygen defects at 1×10¹⁸/cm³ ormore can also be used.

As the second oxide semiconductor, a variety of oxides can be used. Forexample, a material which includes elements selected from indium,gallium, aluminum, zinc, and oxygen at 90 at. % or more, preferably 95at. % or more of the all elements and whose nominal composition formulais represented by In_(a)Ga_(b)Al_(c)Zn_(d)O_(e) (here, a+b+c+d=2, b≧1,2.5<e<3.5) may be used. In order to form the i-type second oxidesemiconductor, the concentration of indium or zinc is preferably lowerthan that of aluminum; c is preferably larger than a, further preferablylarger than 10a, and c is preferably larger than d, further preferablylarger than 10d. In addition, the bandgap of the second oxidesemiconductor is preferably 6 eV or less.

In the case where the first oxide semiconductor and the second oxidesemiconductor are the materials having the above-described compositions,the bandgap of the second oxide semiconductor is wider than that of thefirst oxide semiconductor.

Further, since the first oxide semiconductor with the above-describedcomposition is n-type, the Fermi level is substantially the same levelas the bottom of the conduction band. Therefore, the energy differencebetween the vacuum level and the Fermi level is almost the same as theelectron affinity of the first oxide semiconductor.

On the other hand, since the second oxide semiconductor is i-type, theFermi level is located substantially midway between the conduction bandand the valence band. In the case where the first oxide semiconductorand the second oxide semiconductor are the materials having theabove-described compositions, the following relation is satisfied: thework function of the second oxide semiconductor is higher than theelectron affinity of the first oxide semiconductor.

In an embodiment of the present invention as illustrated in FIG. 1A, afirst semiconductor layer 1 including the first oxide semiconductor issandwiched between a second semiconductor layer 2 including the secondoxide semiconductor and an insulating film 4 that also functions as agate insulating film. Here, the first semiconductor layer 1 needs to bein contact with the second semiconductor layer 2. The firstsemiconductor layer 1 need not necessarily be in contact with theinsulating film 4, but an effect described later can be obtained whenthe first semiconductor layer 1 is in contact with the insulating film4.

Further, the insulating film 4 is sandwiched between a conductive layer5 functioning as a gate and the first semiconductor layer 1 and alsofunctions as a gate insulating film. In addition, a first electrode 3 aand a second electrode 3 b functioning as a source and a drain electrodeare provided in contact with the first semiconductor layer 1.

As the first oxide semiconductor and the second oxide semiconductor, theabove-described materials may be used. The thickness of the firstsemiconductor layer 1 may be more than or equal to 0.1 nm and less thanor equal to 100 nm, and the thickness of the second semiconductor layer2 may be more than or equal to 10 nm and less than or equal to 100 nm.As the insulating film 4, a material which includes elements selectedfrom aluminum, boron, nitrogen, and oxygen at 90 at. % or more,preferably 98 at. % or more of the all elements and whose nominalcomposition formula is represented by Si_(a)Al_(b)B_(c)N_(d)O_(e) (here,it is preferable to satisfy the following relations:0.9<(4a+3b+3c)/(3d+2e)<1.1, b>a) may be used; for example, siliconoxide, silicon oxynitride, aluminum oxide, aluminum oxynitride, aluminumnitride, or the like may be used. In the case where the firstsemiconductor layer 1 is in contact with the insulating film 4, it ispreferable to satisfy the following relation: b>5a.

As illustrated in FIG. 1B, the second semiconductor layer 2 may besandwiched between the first semiconductor layer 1 and an insulatingfilm 6. Here, the insulating film 6 is preferably in contact with thesecond semiconductor layer 2. As the insulating film 6, a material whichincludes elements selected from aluminum, boron, nitrogen, and oxygen at90 at. % or more, preferably 98 at. % or more of the all elements andwhose nominal composition formula is represented bySi_(a)Al_(b)B_(c)N_(d)O_(e) (here, it is preferable to satisfy thefollowing relations: 0.9<(4a+3b+3c)/(3d+2e)<1.1, b>10a, d<5e) may beused; for example, silicon oxide, silicon oxynitride, aluminum oxide,aluminum oxynitride, aluminum nitride, or the like may be used. By usinga material satisfying this condition, the bandgap can be 8 eV or wider.

As illustrated in FIG. 1C, the first semiconductor layer 1 may besandwiched between a second semiconductor layer 2 a and a thirdsemiconductor layer 2 b including a third oxide semiconductor. A pointof FIG. 1C different from points of FIG. 1A is that the thirdsemiconductor layer 2 b is inserted between the first semiconductorlayer 1 and the insulating film 4. Here, the third semiconductor layer 2b need not necessarily be in contact with the insulating film 4 butneeds to be in contact with the first semiconductor layer 1. As thethird oxide semiconductor, a material suitable as the material of thesecond oxide semiconductor may be used, and the same material as that ofthe second oxide semiconductor may be used. Further, the thickness ofthe third semiconductor layer 2 b may be more than or equal to 0.1 nmand less than or equal to 100 nm, preferably more than or equal to 0.1nm and less than or equal to 20 nm.

In an embodiment of the present invention as illustrated in FIG. 2A, thefirst semiconductor layer 1 including the first oxide semiconductor issandwiched between the second semiconductor layer 2 including the secondoxide semiconductor and the insulating film 4 that also functions as agate insulating film. Here, the first semiconductor layer 1 need notnecessarily be in contact with the insulating film 4 but needs to be incontact with the second semiconductor layer 2.

The insulating film 4 is sandwiched between the conductive layer 5functioning as a gate and the first semiconductor layer 1 and alsofunctions as a gate insulating film. In addition, the firstsemiconductor layer 1 is in contact with regions having a conductivitythat is increased by subjecting the first oxide semiconductor to dopingtreatment (doped regions 8 a and 8 b). The doped regions 8 a and 8 b areprovided with the first electrode 3 a and the second electrode 3 bfunctioning as a source and a drain electrode.

Regions 7 a and 7 b formed by doping the second oxide semiconductor areprovided in contact with the second semiconductor layer 2. Theabove-described materials may be used as materials of the first oxidesemiconductor, the second oxide semiconductor, and the insulating film4. Further, the thickness of the first semiconductor layer 1 and thethickness of the second semiconductor layer 2 may be in theabove-described respective ranges.

As illustrated in FIG. 2B, the first electrode 3 a and the secondelectrode 3 b may be provided on surfaces of the doped regions 8 a and 8b opposite to surfaces provided with the insulating film 4. If the dopedregions 7 a and 7 b have enough conductivity, the first electrode 3 aand the second electrode 3 b may be provided in contact with the dopedregions 7 a and 7 b. Further alternatively, a structure in which thefirst semiconductor layer 1 is sandwiched between the secondsemiconductor layer 2 and a third semiconductor layer (not shown)including a third oxide semiconductor may be employed.

Further alternatively, as illustrated in FIG. 2C, the secondsemiconductor layer 2 may be sandwiched between the first semiconductorlayer 1 and the insulating film 6. Here, the insulating film 6 ispreferably in contact with the second semiconductor layer 2. As theinsulating film 6, the above-described material may be used.

In FIGS. 2A to 2C, the doped regions 7 a, 7 b, 8 a, and 8 b need not beprovided if a specific condition is satisfied. For example, asillustrated in FIG. 2D, if the distance x between the first electrode 3a and the conductive layer 5 (or the distance between the secondelectrode 3 b and the conductive layer 5) is 50 nm or less, the dopedregions 7 a, 7 b, 8 a, and 8 b need not be provided.

The reason why a FET having a normally off characteristic or acharacteristic close to the normally off characteristic is obtained byemploying the above-described structures will be described withreference to FIGS. 3A to 3C. First, the junction between the first oxidesemiconductor and the second oxide semiconductor which satisfy theabove-described conditions will be considered. FIG. 3A shows a state(band diagram) of the first oxide semiconductor and a state (banddiagram) of the second oxide semiconductor before they are joinedtogether.

In FIG. 3A, the left diagram is a band diagram of the first oxidesemiconductor, and the right diagram is that of the second oxidesemiconductor. The first oxide semiconductor is typically an n-typesemiconductor such as indium oxide, in which electrons serving ascarriers are supplied to the conduction band, so that the Fermi level islocated just below the conduction band. In the diagram, the energydifference between the conduction band and the Fermi level isexaggerated but is actually only several millivolts. It is analyzed thatin some cases the Fermi level may be located above the bottom of theconduction band. The bandgap of indium oxide is approximately 3.7 eV.

The second oxide semiconductor is typically an i-type semiconductorhaving a wide bandgap, such as gallium oxide, in which almost nocarriers exist, so that the Fermi level is located substantially midwaybetween the valence band and the conduction band. It is observed thatthe bandgap of single-crystal gallium oxide is 4.8 eV, while the bandgapof a thin film of amorphous gallium oxide is 4.2 eV.

Here, the electron affinity (the energy difference between the vacuumlevel and the bottom of the conduction band) of the first oxidesemiconductor is required to be higher than the electron affinity of thesecond oxide semiconductor. Preferably, the difference between theformer electron affinity and the latter electron affinity is 0.3 eV ormore. For example, the electron affinity of indium oxide is 4.8 eV,while the electron affinity of single-crystal gallium oxide is 3.5 eVand the electron affinity of amorphous gallium oxide is 4.3 eV to 4.5eV. Thus, the electron affinity of indium oxide is higher than that ofgallium oxide, and the difference therebetween is 0.3 eV or more.

In addition, the work function of the second oxide semiconductor ispreferably higher than the electron affinity of the first oxidesemiconductor. Preferably, the difference between the former workfunction and the latter electron affinity is 0.5 eV or more. Forexample, the work function of single-crystal gallium oxide is 5.5 eV andthe work function of amorphous gallium oxide is 6.4 eV to 6.6 eV; bothare higher than the electron affinity of indium oxide, and thedifference therebetween is 0.7 eV or more.

When the first oxide semiconductor and the second oxide semiconductorwhich have different properties as described above are joined together,carriers move so as to make the Fermi levels of the first oxidesemiconductor and the second oxide semiconductor located in the samelevel; as a result, the band is bent in a vicinity of the junction asillustrated in FIG. 3B. In other words, in the vicinity of the junction,the conduction band of the first oxide semiconductor becomes apart fromthe Fermi level and the valence band becomes closer to the Fermi level.Such a region in a state different from the original state is called atransition region. At a more remote place from the junction surface, theband state becomes closer to that of original characteristics of thefirst oxide semiconductor or the second oxide semiconductor.

In FIG. 3B, although the bend of the band is shown by a straight lineonly in the transition region, the influence of the carrier movement inpractice reaches a considerable distance and the bend of the band is notlinear. However, properties are significantly affected in a region nearthe junction surface. Thus, semiconductor properties in regions otherthan the transition region may be regarded as their original properties.

The width of the transition region depends on the electron affinities,the bandgaps, and the dielectric constants of the first oxidesemiconductor and the second oxide semiconductor, the concentration ofelectrons in the first oxide semiconductor, and the like. For example,assuming that the first oxide semiconductor is indium oxide with anelectron concentration of 1×10¹⁸/cm³ and the second oxide semiconductoris i-type gallium oxide, a region regarded as a transition region is aportion within about 50 nm on the first oxide semiconductor side fromthe junction surface.

This transition region is formed in the following manner: electrons inthe vicinity of the junction surface in the first oxide semiconductormove and the electron concentration therein is reduced, so that theregion is depleted. Thus, a portion close to the junction surface in thetransition region has a low electron concentration and is quasi-i-type.In addition, since hardly any carriers (electrons) exist in the secondoxide semiconductor, movement of electrons in that region can beignored. The bend of the band is caused mainly in the first oxidesemiconductor.

For example, in the example of FIG. 3B, the energy difference betweenthe bottom of the conduction band of the first oxide semiconductor andthe Fermi level at the junction surface is approximately 1.3 eV. Thisenergy difference is large enough to ignore electrons thermally excitedat room temperature. That is, the electron concentration is extremelylow in the vicinity of the junction surface.

This bend of the band depends on the difference between the workfunction of the second semiconductor layer 2 and the electron affinityof the first semiconductor layer 1. It is preferable that the differenceobtained by subtracting the latter electron affinity from the formerwork function be 0.5 eV or more, and further preferable that thedifference between the former work function and the latter electronaffinity be 1 eV or more.

In the case where the electron affinity of the first oxide semiconductoris higher than the electron affinity of the second oxide semiconductor,as illustrated in FIG. 3B, a discontinuous point (a gap or a step) isgenerated in the conduction band at the junction surface between thefirst oxide semiconductor and the second oxide semiconductor. Thisdiscontinuous point makes it difficult for electrons in the first oxidesemiconductor to move into the second oxide semiconductor when the firstoxide semiconductor is used as a channel of a FET. Accordingly, in thecase where particularly the vicinity of the junction surface in thetransition region is used as a channel, flow of electrons into thesecond oxide semiconductor need not be considered.

If it is assumed that a chemical reaction is not caused at the junctionsurface of this transition region, it can be considered that mostproperties of the first oxide semiconductor including field effectmobility except for low electron concentration are maintained.Accordingly, in the case of using a material having high field effectmobility as the first oxide semiconductor, a characteristic of lowelectron concentration and high field effect mobility in the transitionregion can be obtained.

In FIG. 3B, an example in which the first oxide semiconductor has anenough thickness is described. Even if the first oxide semiconductor isthinned to a thickness that is the same as or smaller than the thicknessof the transition region, a difference is not made and a quasi-i-typeregion is formed in the vicinity of the junction surface.

In other words, by making the thickness of the first oxide semiconductorthe same as or smaller than the thickness of the transition region, theconcentration of electrons in the first oxide semiconductor can bereduced. In addition, high field effect mobility originating from thefirst oxide semiconductor can be obtained. Therefore, a FET manufacturedusing the above-described structure can achieve a high field effectmobility and a normally off characteristic or a characteristic close tothe normally off characteristic.

FIG. 3C is a schematic band diagram along a cross section from Point Ato Point B in the FET of FIG. 1B. The case to be described here is acase in which indium oxide is used as the first oxide semiconductorforming the first semiconductor layer 1, gallium oxide is used as thesecond oxide semiconductor forming the second semiconductor layer 2,aluminum oxide is used for the insulating film 4 and the insulating film6, and tungsten is used for the conductive layer 5.

As shown in FIG. 3C, even when the first semiconductor layer 1 is formedusing an n-type oxide semiconductor such as indium oxide, most part ofthe first semiconductor layer 1 can be a quasi-i-type region. Theelectron concentration in the quasi-i-type region is difficult toobserve directly but is calculated to be 1×10¹⁵/cm³ or lower.Accordingly, the FET having such a structure can have a sufficientlyhigh threshold voltage. In other words, a FET having a normally offcharacteristic or a characteristic close to the normally offcharacteristic can be obtained.

With careful attention to FIG. 3C, the bend of the band in the firstsemiconductor layer 1 is also observed in the vicinity of the interfacewith the insulating film 4. This is caused for the same reason why thetransition region is generated near the junction surface between galliumoxide and indium oxide. In a FET having the bend of the band like this,carriers flow in the vicinity of the bottom of the conduction band.Accordingly, carriers flow in a portion that is some distance (typicallya distance of more than or equal to 1 nm and less than or equal to 10nm) from the interface between the first semiconductor layer 1 and theinsulating film 4.

In a normal MISFET, a trap level or the like is generated at aninterface between a gate insulating film and a semiconductor to reduceFET characteristics; however, the influence of the interface can bereduced with a structure in which carriers flow in a portion that isdistant from the gate insulating film (a buried channel structure). Forthe same reason, in the FET having the structure whose band diagram isshown in FIG. 3C, the influence of the interface between the insulatingfilm 4 and the first semiconductor layer 1 can be reduced.

This bend of the band depends on the difference between the workfunction of the insulating film 4 (since the insulating film 4 isnormally regarded as being i-type, the work function thereof correspondsto the difference between the vacuum level and the Fermi level) and theelectron affinity of the first semiconductor layer 1. For causing a bendlike the one shown in FIG. 3C, it is preferable that the former workfunction be higher than the latter electron affinity, and furtherpreferable that the difference between the former work function and thelatter electron affinity be 1 eV or more.

The electron affinity of n-type indium oxide is approximately 4.8 eV,while the work function of aluminum oxide is 5.7 eV and the workfunction of silicon oxide is 5.1 eV. Thus, aluminum oxide is moreappropriate for the above-described condition. Further, the workfunction of amorphous gallium oxide is 6.4 eV to 6.6 eV and higher thanthe electron affinity of indium oxide by 1.6 eV to 1.8 eV, which ispreferable. Therefore, as illustrated in FIG. 1C, the firstsemiconductor layer 1 of indium oxide or the like may be sandwichedbetween the second semiconductor layer 2 a and the third semiconductorlayer 2 b of gallium oxide or the like.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C illustrate examples of a FET of the present invention;

FIGS. 2A to 2D illustrate examples of a FET of the present invention;

FIGS. 3A to 3C illustrate a principle of a FET of the present invention;

FIGS. 4A to 4F illustrate a manufacturing process of a FET of Embodiment1;

FIGS. 5A to 5E illustrate a manufacturing process of a FET of Embodiment2;

FIGS. 6A to 6D illustrate a manufacturing process of a FET of Embodiment3; and

FIGS. 7A and 7B illustrate examples of a conventional FET.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described below with reference to drawings. However,the embodiments can be implemented in various modes. It will be readilyunderstood by those skilled in the art that modes and details can bechanged in various ways without departing from the spirit and scope ofthe present invention. Therefore, this invention is not interpreted asbeing limited to the description of the embodiments below. Note that instructures of the invention described below, the same portions orportions having similar functions are denoted by the same referencenumerals throughout the drawings, and description thereof is notrepeated.

(Embodiment 1)

In this embodiment, a manufacturing method of a FET will be describedwith reference to FIGS. 4A to 4F. First, as illustrated in FIG. 4A, aconductive layer 102 is formed over a substrate 101. A variety ofsubstrates can be given as examples of the substrate 101, but thesubstrate 101 needs to have such a property as to withstand thesubsequent treatment. Further, it is preferable that a surface of thesubstrate 101 have an insulating property. Accordingly, the substrate101 is preferably an insulator alone; an insulator, metal, orsemiconductor whose surface is provided with an insulating layer; or thelike.

As the insulator, various kinds of glasses, sapphire, quartz, ceramics,plastics, or the like can be used. As the metal, aluminum, copper,stainless steel, silver, or the like can be used. As the semiconductor,silicon, germanium, silicon carbide, gallium nitride, or the like can beused. In this embodiment, barium borosilicate glass is used as thesubstrate 101.

Part of the conductive layer 102 functions as a gate, and a material ofthe conductive layer 102 may be a metal having a high work function suchas platinum, gold, or tungsten. The conductive layer 102 may include oneof the above-described materials alone or may have a multilayerstructure where a portion in contact with a surface of a semiconductorlayer provided later includes any of the above-described materials. Inthis embodiment, a 100-nm-thick tungsten film is formed by a sputteringmethod over a 100-nm-thick titanium film and etched to form theconductive layer 102.

Next, as illustrated in FIG. 4B, an insulating film 103 and a firstoxide semiconductor film 104 are formed. The insulating film 103 alsofunctions as a gate insulating film. For example, silicon oxide,aluminum oxide, silicon oxynitride, aluminum nitride, aluminumoxynitride, hafnium oxide, lanthanum oxide, yttrium oxide, or the likemay be used. The thickness of the insulating film 103 is mainlydetermined in consideration of the process circumstances and the voltageused for the FET but is preferably as small as possible for obtaining anormally-off FET characteristic or a characteristic close to thenormally-off characteristic. For example, the thickness may be more thanor equal to 10 nm and less than or equal to 200 nm. A CVD method or asputtering method is used as a film formation method of the insulatingfilm 103; it is preferable to reduce entry of hydrogen into the film asmuch as possible.

As a material of the first oxide semiconductor film 104, indium oxide isused. Of course, a different kind of oxide semiconductor includingindium as a main component may be used. A film formation method of thefirst oxide semiconductor film 104 may be a sputtering method. Further,it is preferable to reduce entry of hydrogen into the film as much aspossible. The thickness may be more than or equal to 10 nm and less thanor equal to 50 nm. Note the first oxide semiconductor film 104 ispreferably formed without exposure of a surface of the insulating film103 to the air in terms of improving the cleanliness of an interfacebetween the insulating film 103 and the first oxide semiconductor film104.

A second oxide semiconductor film is formed over the first oxidesemiconductor film 104. As a material of the second oxide semiconductorfilm, gallium oxide is used. Of course, a different kind of oxidesemiconductor including gallium as a main component may be used. A filmformation method of the second oxide semiconductor film may be asputtering method. Further, it is preferable to reduce entry of hydrogeninto the film as much as possible. By the analysis of the composition ofthe gallium oxide film formed by a sputtering method by Rutherfordbackscattering spectrometry, such a result that an excessive amount ofoxygen exists in the nominal composition is obtained. That is, inGa₂O_(3+x), x is more than or equal to 0.01 and less than or equal to0.15.

The thickness of the second oxide semiconductor film may be more than orequal to 10 nm and less than or equal to 100 nm. A film of a materialthat can serve as an etching stopper may be provided over the secondoxide semiconductor film. Alternatively, an insulating film serving asan interlayer insulator may be provided over the second oxidesemiconductor film.

Note that the second oxide semiconductor film is preferably formedwithout exposure of a surface of the first oxide semiconductor film 104to the air in terms of improving the cleanliness of an interface betweenthe first oxide semiconductor film 104 and the second oxidesemiconductor film. Further, it is preferable that the insulating film103, the first oxide semiconductor film 104, and the second oxidesemiconductor film be successively formed in terms of improving thecleanliness of the interfaces therebetween.

Then, the second oxide semiconductor film is selectively etched to beformed into an island-shaped second semiconductor layer 105. Part of thesecond semiconductor layer 105 is preferably formed over the conductivelayer 102 as illustrated in FIG. 4C. Although not shown, another part ofthe second semiconductor layer 105 may be provided so as not to overlapwith the conductive layer 102. Further, if the second semiconductorlayer 105 is provided in a region where the conductive layer 102 crossesan upper-layer wiring, the second semiconductor layer 105 also functionsas an interlayer insulator.

Alternatively, in the case where a thick insulating film is additionallyformed over the second oxide semiconductor film and left over the secondsemiconductor layer 105, the thick insulating film can be used as aninterlayer insulator in a region where the conductive layer 102 crossesan upper-layer wiring.

The second semiconductor layer 105 also functions as an etching stopper.Note that an alkaline solution (e.g., an ammonia peroxide mixture or thelike) may be used in etching of the second oxide semiconductor film.Since the solubility of indium oxide in an alkaline solution isextremely low, in the case where indium oxide is used as the material ofthe first oxide semiconductor film 104, the second oxide semiconductorfilm can be selectively etched.

Then, a conductive film 106 is formed over the first oxide semiconductorfilm 104 and the second semiconductor layer 105 (see FIG. 4D). Since theconductive film 106 needs to function as a source and a drain electrodeof a FET later, a material of the conductive film 106 is selected to fitthe purpose. For example, titanium, molybdenum, titanium nitride,molybdenum nitride, and tungsten are given. The conductive film 106 mayinclude one of the above-described materials alone or may have amultilayer structure where a portion in contact with the firstsemiconductor layer includes any of the above-described materials.

Then, the conductive film 106 is selectively etched, so that aconductive layer 106 a and a conductive layer 106 b are formed. Thisetching may be either wet etching or dry etching. In either case, it isnecessary that the second semiconductor layer 105 is not excessivelyetched. For that purpose, an etching stopper is preferably provided overthe second semiconductor layer 105. At the time of this etching, thefirst oxide semiconductor film 104 may be etched.

In the case where the first oxide semiconductor film 104 is not etchedin the above etching, etching is continuously performed using adifferent etching method or the like. At this time, such a condition asto make the etching rate of the first oxide semiconductor film 104higher than that of the second semiconductor layer 105 is employed. Forexample, if wet etching is employed as this etching, an etchantcontaining an oxalic acid or a phosphoric acid may be used. In thismanner, a first semiconductor layer 104 a having an island shape isformed as illustrated in FIG. 4E.

Then, as illustrated in FIG. 4F, an oxide insulating film 107 is formedover an entire surface. As a material of the oxide insulating film 107,silicon oxide, aluminum oxide, silicon oxynitride, aluminum nitride,aluminum oxynitride, hafnium oxide, lanthanum oxide, yttrium oxide, orthe like may be used. The thickness can be more than or equal to 10 nmand less than or equal to 1 μm. In the case where the oxide insulatingfilm 107 needs to function as a protective film, it is preferable toform the oxide insulating film 107 thick.

(Embodiment 2)

In this embodiment, a manufacturing method of a FET will be describedwith reference to FIGS. 5A to 5E. As illustrated in FIG. 5A, aconductive film 202, an insulating film 203, a first oxide semiconductorfilm 204, a second oxide semiconductor film 205, and an oxide insulatingfilm 206 are stacked over a substrate 201. Formation of these films ispreferably performed successively without exposure to the air on the wayin order to keep interfaces of the films clean. In addition, successivefilm formation is also effective in order to make the hydrogenconcentration in the multi-layer films lower than or equal to 1×10¹⁸atoms/cm³, preferably lower than or equal to 1×10¹⁶ atoms/cm³.

The substrate 201, the conductive film 202, the insulating film 203, thefirst oxide semiconductor film 204, the second oxide semiconductor film205, and the oxide insulating film 206 may be formed using materials,thicknesses, and formation methods that are described in Embodiment 1 asbeing suitable for the substrate 101, the conductive layer 102, theinsulating film 103, the first oxide semiconductor film 104, the secondoxide semiconductor film, and the oxide insulating film 107,respectively.

Then, a resist is applied over the oxide insulating film 206, and aresist mask 207 having two levels of thicknesses is formed using amulti-tone mask as illustrated in FIG. 5A.

With the use of the resist mask 207, the conductive film 202, theinsulating film 203, the first oxide semiconductor film 204, the secondoxide semiconductor film 205, and the oxide insulating film 206 areetched, so that a conductive layer 202 a, an insulating layer 203 a, afirst semiconductor layer 204 a, a second semiconductor layer 205 a, andan oxide insulating layer 206 a are formed (see FIG. 5B). An anisotropicdry etching method is preferable as this etching, but a wet etchingmethod or an isotropic dry etching method may also be used.

Next, the resist mask 207 is subjected to ashing to be reduced inthickness, so that a resist mask 207 a is formed (see FIG. 5C).

Then, with the use of the resist mask 207 a, the insulating layer 203 a,the first semiconductor layer 204 a, the second semiconductor layer 205a, and the oxide insulating layer 206 a are etched, so that aninsulating layer 203 b, a first semiconductor layer 204 b, a secondsemiconductor layer 205 b, and an oxide insulating layer 206 b whicheach have almost the same shape and form an island shape are formed (seeFIG. 5D). As this etching, an anisotropic dry etching method ispreferably used, but a wet etching method or an isotropic dry etchingmethod may also be used.

The conductive layer 202 a need not necessarily be etched; however, itis difficult to find a condition under which the conductive layer 202 ais not etched at all at the time of the above-described etching.Accordingly, although the etched amount varies, the conductive layer 202a is etched into a conductive layer 202 b having a surface part of whichis etched. In particular, in the case where a sufficiently high etchingselectivity cannot be obtained, it is preferable to form the conductivefilm 202 sufficiently thick. For example, the thickness of theconductive film 202 may be more than or equal to 200 nm and less than orequal to 1 μm.

The thickness of the conductive film 202 may be set to be more than orequal to 50% and less than or equal to 500% of the sum of thethicknesses of the insulating film 203, the first oxide semiconductorfilm 204, the second oxide semiconductor film 205, and the oxideinsulating film 206. When the thickness of the conductive film 202 has asufficient thickness in this manner, a necessary thickness can besecured even if overetching is caused to some extent.

In this embodiment, even if the conductive film 202 is thick, theinfluence on the shape of a thin film to be stacked later is small, andthere is rather an advantage that the resistance of the conductive layer202 b is reduced.

Then, the resist mask 207 a is removed. For this removal, a stripper maybe used; the concentration of hydrogen ions in the selected stripperneeds to be appropriate because gallium oxide has a property of beingsoluble in an alkaline solution.

Alternatively, the resist mask 207 a may be removed by an ashing method.In the case of employing an ashing method, a residue may become aproblem. However, in this embodiment, even if a residue is generated,the residue exists over the oxide insulating layer 206 b. Therefore, bymaking the oxide insulating layer 206 b sufficiently thick, an influenceon FET characteristics can be reduced. Since the oxide insulating layer206 b has a function as an interlayer insulator as well in thisembodiment, it is particularly preferable to form the oxide insulatinglayer 206 b thick.

Then, an interlayer insulator 208 having a flat surface is formed, andthe interlayer insulator 208, the oxide insulating layer 206 b, and thesecond semiconductor layer 205 b are etched to form an opening reachingthe first semiconductor layer 204 b. The opening is provided so as tooverlap with the insulating layer 203 b. The opening is preferablyprovided to have a perimeter that is 200 nm or more, preferably 1 μm ormore away from a perimeter of the insulating layer 203 b.

The opening can be formed by either a wet etching method or a dryetching method. Note that in either case of etching, the firstsemiconductor layer 204 b may be chemically influenced by the etching.

Next, a conductive film is formed and etched into a desired shape, sothat a conductive layer 209 a and a conductive layer 209 b are formed.The conductive film may be formed using a material, a thickness, and afilm formation method that are suitable for the conductive film 106 ofEmbodiment 1.

Portions in contact with the first semiconductor layer 204 b of theconductive layers 209 a and 209 b function as a source electrode and adrain electrode of a FET. This stage is illustrated in FIG. 5E. Thedistance between the portions in contact with the first semiconductorlayer 204 b of the conductive layers 209 a and 209 b is the channellength of the FET.

Note that in the case where high temperature treatment of 400° C. orhigher is performed after the conductive layers 209 a and 209 b areformed, an element included in the conductive layers 209 a and 209 b maydiffuse into the first semiconductor layer 204 b and adversely affectcharacteristics of the first semiconductor layer 204 b. Accordingly,after the conductive layers 209 a and 209 b are formed, it is preferableto avoid such high-temperature treatment. Note that since the conductivelayers 209 a and 209 b are formed at the final stage of themanufacturing process of the FET, high-temperature treatment is hardlynecessary after the formation of the conductive layers 209 a and 209 b.

Moreover, in many cases, characteristics of an edge region (a perimeterregion) of the first semiconductor layer 204 b are not preferable. Thisregion often has higher conductivity than the other region and can be afactor of a leak current in a FET having a shape like the oneillustrated in FIG. 4F. This is because in the FET having the shape likethe one illustrated in FIG. 4F, an edge region of the firstsemiconductor layer 104 a is in contact with the conductive layer 106 aand the conductive layer 106 b functioning as a source and a drainelectrode.

However, in the FET illustrated in FIG. 5E, the conductive layers 209 aand 209 b functioning as a source and a drain electrode do not overlapwith the edge region of the first semiconductor layer 204 b as describedabove. Therefore, if the edge region of the first semiconductor layer204 b has a high conductivity, a leakage current does not flow betweenthe conductive layers 209 a and 209 b. Thus, a FET having a sufficientlysmall off-state current can be obtained.

As is clear from FIG. 5E, the first semiconductor layer 204 b is formedover a flat surface. For example, a step of a semiconductor layer as inthe first semiconductor layer 104 a of FIG. 4F affects FETcharacteristics. Since elements having such a step are difficult to beformed uniformly, this may cause a variation in FET characteristics.

For example, in FIG. 4E, the conductive layers 106 a and 106 b areprovided to be bilaterally symmetrical about the first semiconductorlayer 104 a and the conductive layer 102; however, it is difficult toform all the elements so as to have such a shape. Even if a slightparallel displacement of the conductive layers 106 a and 106 b to theright side (or the left side) in the drawing occurs owing to an error inmask alignment, positional relations between the conductive layers 106 aand 106 b and a curved portion of the first semiconductor layer 104 abecome different, which may change characteristics of the FET in somecases. That is, variation in FET characteristics may arise.

In particular, when the first semiconductor layer contains a certaincrystalline component, the variation tends to be large. This is becausethere are large differences in the crystal orientation, size ofcrystals, and the like between a flat portion and a curved portion ofthe first semiconductor layer. In the case of indium oxide, such acrystalline component is generated even when indium oxide is annealed ata relatively low temperature.

By contrast, it is easy to form a flat semiconductor layer uniformlyover a flat surface. Even when a slight parallel displacement of theconductive layers 209 a and 209 b in FIG. 5E to the right side (or theleft side) in the drawing occurs owing to an error in mask alignment,positional relations between the first semiconductor layer 204 b and theconductive layers 209 a and 209 b are not changed. That is, variation inFET characteristics is less.

Therefore, variation of characteristics of the FET described in thisembodiment is less than that of the FET including a large step of asemiconductor layer. Such a FET is favorably used in a circuit whosevariation in threshold voltage needs to be small.

For example, in an active-matrix organic electroluminescent displaydevice, variation in threshold voltage of driving transistors ispreferably small in order to prevent display unevenness. In addition, aFET exhibiting high field effect mobility is preferable as a drivingtransistor in order to reduce resistance loss. In order to achieve suchobjects, a FET which has the structure illustrated in FIG. 5E and ismanufactured so as to include an oxide semiconductor including indium asa main component in a semiconductor layer is suitable. Such a FETexhibits, for example, a field effect mobility of 50 cm²/Vs or higher,preferably 70 cm²/Vs or higher.

Further in this embodiment, the first semiconductor layer 204 b and theinsulating layer 203 b are provided over flat surfaces; there is no needto consider step coverage or the like of these thin films. Accordingly,the first semiconductor layer 204 b and the insulating layer 203 b canbe made as thin as possible.

Thinning of the first semiconductor layer 204 b and the insulating layer203 b is effective for suppressing a negative shift of the thresholdvoltage of the FET and reducing the off-state current. In particular, ina FET having a channel length of 0.3 μm or less, the first semiconductorlayer 204 b and the insulating layer 203 b are required to be thin.

In order to suppress the negative shift of the threshold voltage of theFET, the following relation needs to be satisfied: (the channellength)>5×(the thickness of the first semiconductor layer 204 b+(thethickness of the insulating layer 203 b)×(the dielectric constantratio)). Here, the dielectric constant ratio is a value obtained bydividing the dielectric constant of the first semiconductor layer 204 bby the dielectric constant of the insulating layer 203 b.

For example, in the case where the channel length is 0.3 μm, the firstsemiconductor layer 204 b is formed of indium oxide (dielectricconstant: 18) to have a thickness of 30 nm, and the insulating layer 203b is formed of silicon oxide (dielectric constant: 4), the thickness ofthe insulating layer 203 b needs to be 10 nm or less.

It is difficult to form such a thin film over an uneven surface withfavorable coverage, resulting in yield reduction. On the other hand, itis easy to form such a thin film over a flat surface. In this respect, aFET of the structure described in this embodiment is advantageous.

Note that according to the manufacturing process described in thisembodiment, the number of times of mask alignment is 2, which is lessthan that in the method illustrated in FIGS. 4A to 4F (3 times of maskalignment). Thus, the manufacturing process described in this embodimentis effective in reducing the possibility of defects caused bymisalignment and improving yields.

(Embodiment 3)

In this embodiment, a manufacturing method of a FET will be describedwith reference to FIGS. 6A to 6D. As illustrated in FIG. 6A, an oxideinsulating film 302, a second oxide semiconductor film 303, a firstoxide semiconductor film 304, and an insulating film 305 are stackedover a substrate 301. Formation of theses films is preferably performedsuccessively without exposure to the air on the way in order to keepinterfaces of the films clean.

For example, a silicon wafer may be used as the substrate 301, a200-nm-thick silicon oxide film may be used as the oxide insulating film302, a 50-nm-thick gallium oxide film may be used as the second oxidesemiconductor film 303, a 1-nm-thick indium oxide film may be used asthe first oxide semiconductor film 304, and a 2-nm-thick silicon oxidefilm may be used as the insulating film 305. The oxide insulating film302 may be formed by performing thermal oxidation on the substrate 301.The second oxide semiconductor film 303, the first oxide semiconductorfilm 304, and the insulating film 305 may be formed by atomic layerdeposition (ALD). These films may be successively formed in a depositionapparatus.

Then, the second oxide semiconductor film 303, the first oxidesemiconductor film 304, and the insulating film 305 are selectivelyetched into a second semiconductor layer 303 a, a first semiconductorlayer 304 a, and an insulating layer 305 a.

Further, a film of silicon oxide or the like having a thickness of morethan or equal to 100 nm and less than or equal to 500 nm is formed as aninsulator 306 so as to cover a perimeter region of the insulating layer305 a. Then, a conductive layer 307 is formed (see FIG. 5B). A materialand a formation method of the conductive layer 307 may be determined byreference to the description of the conductive layer 102 inEmbodiment 1. The thickness of the conductive layer 307 is preferablymore than or equal to twice the sum of the thickness of the firstsemiconductor layer 304 a and the thickness of the insulating layer 305a.

Although not shown, the conductive layer 307 climbs over the insulator306 in a region over the perimeter region of the insulating layer 305 a.Therefore, the conductive layer 307 is not in direct contact with thefirst semiconductor layer 304 a.

Since the conductive layer 307 serves as a gate of a FET, the width ofthe conductive layer 307 determines the channel length of the FET. Here,the negative shift of the threshold voltage can be suppressed andoff-state current can be reduced when the width of the conductive layer307 is 5 times or more, preferably 10 times or more the sum of thethickness of the first semiconductor layer 304 a and the thickness ofthe insulating layer 305 a multiplied by the dielectric constant ratiobetween the first semiconductor layer 304 a and the insulating layer 305a (the dielectric constant of the first semiconductor layer 304 a/thedielectric constant of the insulating layer 305 a).

For example, since the dielectric constant of indium oxide isapproximately 18 and the dielectric constant of silicon oxide isapproximately 4 (these values vary delicately with the film formationmethod), the dielectric constant ratio is approximately 4.5. By usingthe above-described thicknesses in the calculation, the width of theconductive layer 307 is obtained to be preferably 5 times or more,further preferably 10 times or more of the value obtained by thefollowing formula: (the thickness of the first semiconductor layer 304 a(1 nm))+(the thickness of the insulating layer 305 a (2 nm))×(thedielectric constant ratio (4.5)). That is, the width of the conductivelayer 307 is preferably more than or equal to 50 nm, and furtherpreferably more than or equal to 100 nm.

As is apparent from the above-described calculation, the width of theconductive layer 307 can be small if a high dielectric constant materialis used for the insulating layer 305 a. For example, the width of theconductive layer 307 can be set to 10 nm or more, preferably 20 nm ormore if a material having a dielectric constant of approximately 30 suchas hafnium oxide is used as a material of the insulating layer 305 a.

Unlike a normal MOSFET in which a silicon semiconductor is used, even inthe case where a different kind of material such as hafnium oxide isformed in contact with an oxide semiconductor including indium as a maincomponent, FET characteristics are little affected by a defect level atan interface between the first semiconductor layer 304 a and theinsulating layer 305 a. In particular, in the case of using hafniumoxide, since the work function thereof (5.7 eV) is higher than theelectron affinity of indium oxide (4.8 eV), the band of indium oxide isbent and a buried-channel-like shape is obtained. Accordingly, theinfluence of the interface between the first semiconductor layer 304 aand the insulating layer 305 a becomes less.

Next, irradiation with ions having a high reducing property is performed(see FIG. 6C). The peak of ion energy at this time may be set to bepositioned between the interface of the first semiconductor layer 304 aand the second semiconductor layer 303 a and the interface of the secondsemiconductor layer 303 a and the oxide insulating film 302. Under thiscondition, ions do not pass through the conductive layer 307;accordingly, ions are introduced into the first semiconductor layer 304a and the second semiconductor layer 303 a in a self-aligned mannerusing the conductive layer 307 as a mask.

If ions having a high reducing property are introduced into an oxidesemiconductor including indium oxide as a main component, the ions arecombined with oxygen, so that the oxide semiconductor is reduced. As aresult, the electron concentration in a portion into which the ions areintroduced is increased, so that conductivity is increased. The amountof ions introduced may be determined in accordance with the desiredconductivity. The ions may be included in the first semiconductor layer304 a at 2×10²⁰/cm³ or more.

As the ions having a high reducing property, ions of an element an oxideof which has a stronger binding force than indium oxide, such as boron,carbon, phosphorus, silicon, aluminum, or gallium, may be used. Amongthem, phosphorus and boron are easy to be used because they are used ina normal semiconductor process.

It is to be noted that the hydrogen is not introduced at the same timeas the introduction of the ions. Thus, it is not preferable to use ahydride (e.g., diborane (B₂H₆) or phosphine (PH₃)) as an ion source. Ifhydrogen is introduced into the oxide semiconductor, the electronconcentration is increased; however, the hydrogen moves in the oxidesemiconductor and causes variation in FET characteristics, which leadsto lower reliability. It is desirable that the hydrogen concentration inthe ions be 1 at. % or less.

At the time of this ion introduction, ions are introduced into thesecond semiconductor layer 303 a as well, so that gallium oxide is alsoreduced by the ions. Thus, regions having high conductivity 303 b and303 c are formed in the second semiconductor layer 303 a.

Here, if the thickness of the second semiconductor layer 303 a (that is,the thickness of the second oxide semiconductor film 303) is too small,electric resistance cannot be reduced sufficiently even when theconductivity of the second semiconductor layer 303 a is high. Therefore,the thickness of the second semiconductor layer 303 a is preferably setto an appropriate thickness. The concentration of ions introduced intothe second semiconductor layer 303 a is preferably higher than that ofions introduced into the first semiconductor layer 304 a to form adegenerated semiconductor. For example, ions may be included in thesecond semiconductor layer 303 a at 5×10²⁰/cm³ or more.

Next, for example, a protective insulating film 308 is formed by aplasma CVD method or the like using silicon nitride or the like to havea thickness of more than or equal to 200 nm and less than or equal to 1μm. Here, the protective insulating film 308 may be a film that givessuch a stress as to compress the first semiconductor layer 304 a. Withthis stress, the first semiconductor layer is compressed and thedistance between indium atoms in indium oxide can be shortened. As aresult, a transport property of the first semiconductor layer(typically, field effect mobility) is improved.

Further, silicon oxide or the like is deposited to a thickness of morethan or equal to 200 nm and less than or equal to 2 μm, and planarizedby chemical mechanical polishing to obtain an interlayer insulator 309having a flat surface. Further, the interlayer insulator 309, theprotective insulating film 308, and the insulating layer 305 a areetched to form opening reaching the first semiconductor layer 304 a orthe second semiconductor layer 303 a. Then, a conductive layer 310 a anda conductive layer 310 b are formed to fill the openings (see FIG. 6D).

As described above, since the first semiconductor layer 304 a is 1 nmthick, the openings may penetrate through the first semiconductor layer304 a due to overetching. However, since the conductivity of the secondsemiconductor layer 303 a is enough even in such a case, the conductivelayers 310 a and 310 b function as electrodes of the FET. In otherwords, current flows through the following route: the conductive layer310 a, the region having high conductivity 303 b in the secondsemiconductor layer, the first semiconductor layer 304 a, the regionhaving high conductivity 303 c of the second semiconductor layer, andthe conductive layer 310 b.

Also for the purpose of preventing overetching of the secondsemiconductor layer 303 a, the second semiconductor layer 303 apreferably has an appropriate thickness. Although the secondsemiconductor layer 303 a has a thickness of 50 nm in this embodiment,the thickness of the second semiconductor layer 303 a is preferable aslong as it is 30 nm or more.

(Embodiment 4)

The semiconductor devices described in Embodiments 1 to 3 can be used ina variety of electronic devices. For example, they can be used indisplay devices such as liquid crystal displays, EL displays, and FE(field emission) displays; driver circuits of such display devices;driver circuits of image sensors; semiconductor memories;microprocessors; and the like. Further, the semiconductor devicesdescribed in Embodiments 1 to 3 can be used in a variety of electronicdevices including any of such display devices, for example in televisiondevices, personal computers, communication devices such as mobilephones, electronic notebooks, and portable music players.

EXPLANATION OF REFERENCE

-   1: first semiconductor layer, 2: second semiconductor layer, 2 a:    second semiconductor layer, 2 b: third semiconductor layer, 3 a:    first electrode, 3 b: second electrode, 4: insulating film, 5:    conductive layer, 6: insulating film, 7 a: doped region, 7 b: doped    region, 8 a: doped region, 8 b: doped region, 11: semiconductor    layer, 13 a: source electrode, 13 b: drain electrode, 14: gate    insulating film, 15: gate, 16: protective insulating film, 101:    substrate, 102: conductive layer, 103: insulating film, 104: first    oxide semiconductor film, 104 a: first semiconductor layer, 105:    second semiconductor layer, 106: conductive film, 106 a: conductive    layer, 106 b: conductive layer, 107: oxide insulating film, 201:    substrate, 202: conductive film, 202 a: conductive layer, 202 b:    conductive layer, 203: insulating film, 203 a: insulating layer, 203    b: insulating layer, 204: first oxide semiconductor film, 204 a:    first semiconductor layer, 204 b: first semiconductor layer, 205:    second oxide semiconductor film, 205 a: second semiconductor layer,    205 b: second semiconductor layer, 206: oxide insulating film, 206    a: oxide insulating layer, 206 b: oxide insulating layer, 207:    resist mask, 207 a: resist mask, 208: interlayer insulator, 209 a:    conductive layer, 209 b: conductive layer, 301: substrate, 302:    oxide insulating film, 303: second oxide semiconductor film, 303 a:    second semiconductor layer, 303 b: region having high conductivity,    303 c: region having high conductivity, 304: first oxide    semiconductor film, 304 a: first semiconductor layer, 305:    insulating film, 305 a: insulating layer, 306: insulator, 307:    conductive layer, 308: protective insulating film, 309: interlayer    insulator, 310 a: conductive layer, 310 b: conductive layer.

This application is based on Japanese Patent Application serial no.2010-136705 filed with Japan Patent Office on Jun. 16, 2010, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A field effect transistor comprising: afirst semiconductor layer including a first oxide semiconductor; asecond semiconductor layer including a second oxide semiconductor andbeing in contact with a first surface of the first semiconductor layer;a third semiconductor layer including a third oxide semiconductor andbeing in contact with a second surface of the first semiconductor layer;a conductive layer adjacent to the third semiconductor layer; and aninsulating film between the conductive layer and the third semiconductorlayer, wherein a bandgap of each of the second oxide semiconductor andthe third oxide semiconductor is wider than a bandgap of the first oxidesemiconductor.
 2. The field effect transistor according to claim 1,wherein the first oxide semiconductor comprises at least indium, andwherein a percentage of indium to all elements having an atomic numberof 11 or more is 50 at. % or more in the first oxide semiconductor. 3.The field effect transistor according to claim 1, wherein each of thesecond oxide semiconductor and the third oxide semiconductor comprisesat least gallium, and wherein a percentage of gallium to all elementshaving an atomic number of 11 or more is 50 at. % or more in each of thesecond oxide semiconductor and the third oxide semiconductor.
 4. Thefield effect transistor according to claim 1, wherein each of the secondoxide semiconductor and the third oxide semiconductor is an i-type oxidesemiconductor.
 5. The field effect transistor according to claim 1,wherein the bandgap each of the second oxide semiconductor and the thirdoxide semiconductor is 6 eV or less.
 6. The field effect transistoraccording to claim 1, further comprising a source electrode and a drainelectrode which are in contact with the first surface of the first oxidesemiconductor.
 7. A field effect transistor comprising: a firstsemiconductor layer including a first oxide semiconductor; a secondsemiconductor layer including a second oxide semiconductor and being incontact with a first surface of the first semiconductor layer; a thirdsemiconductor layer including a third oxide semiconductor and being incontact with a second surface of the first semiconductor layer; aconductive layer adjacent to the third semiconductor layer; and aninsulating film between the conductive layer and the third semiconductorlayer, wherein a bandgap of each of the second oxide semiconductor andthe third oxide semiconductor is wider than a bandgap of the first oxidesemiconductor, wherein each of the second oxide semiconductor and thethird oxide semiconductor comprises at least gallium and indium, andwherein a percentage of gallium to all elements other than oxygen is 80at. % or more in each of the second oxide semiconductor and the thirdoxide semiconductor.
 8. The field effect transistor according to claim7, wherein the first oxide semiconductor comprises at least indium, andwherein a percentage of indium to all elements having an atomic numberof 11 or more is 50 at. % or more in the first oxide semiconductor. 9.The field effect transistor according to claim 7, wherein each of thesecond oxide semiconductor and the third oxide semiconductor is ani-type oxide semiconductor.
 10. The field effect transistor according toclaim 7, wherein the bandgap each of the second oxide semiconductor andthe third oxide semiconductor is 6 eV or less.
 11. The field effecttransistor according to claim 7, further comprising a source electrodeand a drain electrode which are in contact with the first surface of thefirst oxide semiconductor.
 12. A field effect transistor comprising: afirst semiconductor layer including a first oxide semiconductor; asecond semiconductor layer including a second oxide semiconductor andbeing in contact with a first surface of the first semiconductor layer;a third semiconductor layer including a third oxide semiconductor andbeing in contact with a second surface of the first semiconductor layer;a conductive layer adjacent to the third semiconductor layer; and aninsulating film between the conductive layer and the third semiconductorlayer, wherein a bandgap of each of the second oxide semiconductor andthe third oxide semiconductor is wider than a bandgap of the first oxidesemiconductor, wherein an energy difference between a vacuum level ofthe second oxide semiconductor and a Fermi level of the second oxidesemiconductor is larger than an energy difference between a vacuum levelof the first oxide semiconductor and a Fermi level of the first oxidesemiconductor, and wherein an energy difference between a vacuum levelof the third oxide semiconductor and a Fermi level of the third oxidesemiconductor is larger than the energy difference between the vacuumlevel of the first oxide semiconductor and the Fermi level of the firstoxide semiconductor.
 13. The field effect transistor according to claim12, wherein the first oxide semiconductor comprises at least indium, andwherein a percentage of indium to all elements having an atomic numberof 11 or more is 50 at. % or more in the first oxide semiconductor. 14.The field effect transistor according to claim 12, wherein each of thesecond oxide semiconductor and the third oxide semiconductor comprisesat least gallium, and wherein a percentage of gallium to all elementshaving an atomic number of 11 or more is 50 at. % or more in each of thesecond oxide semiconductor and the third oxide semiconductor.
 15. Thefield effect transistor according to claim 12, wherein each of thesecond oxide semiconductor and the third oxide semiconductor is ani-type oxide semiconductor.
 16. The field effect transistor according toclaim 12, wherein the bandgap each of the second oxide semiconductor andthe third oxide semiconductor is 6 eV or less.
 17. The field effecttransistor according to claim 12, further comprising a source electrodeand a drain electrode which are in contact with the first surface of thefirst oxide semiconductor.